Properties of Transformer Cores

Laminated Transformer Core

The core of a laminated transformer consists of a stack of punched sheet alloy made of iron and nickel (the laminations). The percentage of nickel is adjusted to give a reduced energy loss when the core is magnetised by the magnetic field produced when the primary winding is energised.

Further improvements are made to the molecular structure by a rolling process. The typical flux density of 0.5mm thick lamination is 1.3 to 1.5 tesla. For grain orientated material a higher flux density is used to minimise the size but incurs an increased cost.

A further process is carried out to insulate the surface of the sheet by heat treatment, either gas or chemical. This is to reduce eddy currents that would flow from lamination to lamination (and lower efficiency).

The punching process causes a slight degradation in the performance and further heat treatment may be required.

Generally three thicknesses of lamination are manufactured, 0.5mm and 0.35mm for 50Hz and up to 400Hz, 0.1mm up to 1000Hz. Higher frequencies can be used but the flux density would have to be reduced to minimise core losses and reduce distortion, for example in amplifier output transformers.

Note: Because of the gaps in the magnetic circuit and the lamination material, the switch on in-rush current is lower than the equivalent power rated toroidal transformer, but they are larger by comparison.

Toroidal Transformer Core

A toroidal core is constructed using a strip of nickel iron material wound in a helix to form a circular core.

The typical thickness suitable for most applications is 0.35mm however 0.1mm is more common for high frequency applications. The process to manufacture the strip is similar to the material used in the laminated transformers. Because no punching of the material is required it has an improved magnetic performance although after the material is cut to width and formed into a roll a further heat treatment is carried out.

The permeability of the wound core is greater than its equivalent laminated transformer which has punching losses and gaps in its magnetic circuit. Due to the improved core characteristics toroids can be run at 1.6 to 1.8 tesla with low core losses and low magnetisation currents. This improved core performance allows the size of toroid to be smaller than the equivalent laminated transformer.

Note: For large toroidal transformers the high permeability and lower core losses may give rise to large inrush currents when switched on. This can be reduced by methods of winding and by reducing flux density or providing soft start circuitry. The transformer may have to be run from class “D” circuit breaker.

Ferrite Transformer Core

The Ferro ceramic material used in the magnetic core can be moulded and fired to give a wide number of core shapes and styles. These include EE, EI, UI, ETD, EC, RM, PM toroid etc. The maximum size is limited but multiples of EE, EI & UI can be used to make larger sizes.

The frequency range has increased over time due to the research into new materials by the core manufacturers and can be used in power circuits up to 5MHz & beyond. For some of the more exotic materials the flux density has to be reduced to minimise core losses.

By changing the material used, i.e. iron powder and other mixtures the characteristics of the core can be changed to give a wide range of permeability values with high DC bias ability in both EI style and toroidial form.